At present headlights of the parabolic mirror type or with a surface capable of self-generation of a cut-off beam (in particular a dipped beam or fog-lamp beam) generally have, at least in the case of the mirror, a width which is considerably greater than the height.
There are a number of considerations which go to explain this. First, motor vehicle manufacturers have a tendency to manufacture vehicles which are ever more aerodynamic, and an important factor in aerodynamic design is a general shape which tapers towards the front with a sloping bonnet, and the height of the space provided at the front of the vehicle, where the headlights are located, is consequently becoming smaller and smaller.
In parallel with this, a good light output requires that a reflecting surface of considerable area be maintained, and because the height of the headlights is reduced, this area can only be obtained in a lateral direction.
Apart from this, obtaining a light beam of good quality, in particular if it is to provide a point of concentration on the axis of the road, requires that the beam be formed with a significant proportion of small images of the light source (typically the filament of an incandescent lamp or the arc of a discharge lamp), and this requires that a mirror be designed with areas which extend as far as possible from the source, although in practice these areas can only extend away from the light source in a lateral direction, for the reasons of overall size previously mentioned.
As a corollary to this, traditional headlights with this type of contour work best with a light source which is oriented axially, this orientation contributing to the overall light yield and, with mirrors which are highly elongate laterally, creating a large proportion of images on a projection screen which are slightly inclined to the horizontal. This is well-suited to obtaining beams with a sharp cut-off and substantial lateral spread.
By contrast with the general trend over a good number of years, there is now a demand from manufacturers for headlights with a width equal to or less than their height.
With headlights of traditional optical design this new type of contour causes a number of problems.
First, with an axial source the resulting substantial height of the mirror above and below the light source will result in a substantial proportion of images strongly inclined to the horizontal, i.e. slightly inclined to the vertical, which in the first place will contribute to deterioration of the quality of the cut-off, secondly will light up the roadway too close to the vehicle and thirdly will cause problems with obtaining a beam of satisfactory width.
A fog-lamp has already been proposed, in particular in document FR-A-2 602 305 in the Applicant's name, which features a transverse light source and a mirror which can have a height greater than its width.
This known headlight nevertheless still has certain disadvantages with regard to the distribution of images of the source. FIG. 1 of the drawings represents a vertical axial section of the reflecting surface described in the above document.
This section is defined in its upper part by a section of a parabola 20h with its focus at a fixed point Fh (or "top focus") behind the filament 10 and in its lower part by another section of a parabola 20b with its focus at another fixed point Fb (or "bottom focus") in front of the filament 10. Such sections inevitably lead to images of the filament which are delimited at the top by a horizontal cut-off defined by the intersection of the y--y axis with a projection screen, as shown at "C" in FIG. 1 (this is the case in particular with an image I1 emitted by the upper part of the mirror), and which are excessively inclined downwards relative to the cut-off C (this is the case in particular with an image I2 produced by a middle area of the lower part of the mirror).
Accordingly, the resulting cut-off offers considerable scope for improvement.
In addition to this, the headlight described in the above document is uniquely well-suited to creating a fog-lamp beam with a flat cut-off and there is nothing in the above document to indicate or suggest a way in which a beam might be created with a more complex cut-off, in particular a dipped beam of the European or American type.
In addition to this, the mirror described above is unable in itself to provide a light beam of any substantial width, and it is therefore impossible to use a lens which has optical elements for spreading the light, something which is frequently desired by motor vehicle stylists, however.